Title: Exotic Explosions and Eruptions: Exploring a New Transient Phase-Space with Pan-STARRS

Abstract:
For nearly a century two primary classes of optical transients - nova
eruptions and supernova explosions - have been studied in detail. These
transients occupy narrow ranges of absolute magnitudes, around -8 and -18 mag
(+/-2 mag) respectively. However, nascent dedicated surveys (e.g. Pan-STARRS)
are now revealing new classes of optical transients in the wide nova-SN gap
and at very high luminosities (<-20 mag). The origin of these events is hotly
debated: they are argued to represent massive star eruptions, deficient white
dwarf thermonuclear explosions, eta Carina-like ejections, and possibly new
SN mechanisms (electron-capture, fallback, pair-instability). The Pan-STARRS
project provides an unprecedented opportunity to explore these new phenomena
thanks to its unmatched depth and multi-band photometry. Here we propose to
continue our successful TOO spectroscopy of Pan-STARRS transients in the
nova-SN gap and at high luminosity to classify and characterize their origin
for the first time. Our Gemini program already uncovered some of the most
luminous transients to date (-21 to -24 mag at z=0.5-2), and demonstrated the
potential of these events to probe the ISM of high-redshift galaxies (much
like GRBs). Pan-STARRS will continue to discover many such transients and
Gemini spectroscopy will determine their origin.